In connection with systems wherein information is transmitted by means of light, the technique of connecting optical fibers with various optical elements, that is, the optical connection technique, has been under research and development. Thus, a variety of optical connection devices have been proposed.
FIGS. 1 and 2 are schematic illustrations of the constructions of typical examples of conventional optical connection devices such as optical connectors for use in such optical connection.
FIG. 1 is a plan view of an optical connector with two cores, which is disclosed in Japanese Utility Model Application Laid-Open No. 4,406/1989. This optical connector is of such a construction that a pair of assemblies each having tips of single optical cables 10a and 10b (the tips are usually parts consisting of a core portion and a clad portion except for a covering for a core wire, and are also referred to as "terminal strands of optical fibers,") inserted into respective ferrule units 12a and 12b (in a cylindrical form) are disposed and fixed in a housing 14 with such a distance therebetween that the ferrule units 12a and 12b cannot come into contact with each other.
FIG. 2 is a perspective view of the plug of a multi-core optical fiber connector as disclosed in Japanese Patent Application Laid-Open No. 72,912/1983. A number of optical fibers 16a, 16a, 16c and 16d have the terminal strands thereof inserted into respective ferrule units 18a,18b, 18c and 18d, which are attached to a plug body 22 in such a way as to pass through corresponding through-holes 20a, 20b, 20c and 20d provided in a plug base. In this conventional example as well, the ferrule units are disposed with such a distance therebetween that they cannot come into contact with each other.
Meanwhile, the core diameter of the core wires of optical fibers is usually in the range of about 4 .mu.m to about 50 .mu.m, while the outer diameter of clad portions around cores is generally about 100 .mu.m. Ferrule units to highly accurately position and fix the terminal strands of optical fibers having such an outer diameter include metallic ferrules, ceramic ferrules, and plastic ferrules, among which ceramic ferrules are now predominantly used since they are especially excellent in strength, final processing accuracy, connection characteristics, etc. All of such ferrules are usually in a cylindrical form.
In every one of these ferrules, a very small through-hole is provided to allow the terminal strand of an optical fiber having an axis in common therewith to be inserted and fixed thereinto. In the case of zirconia ceramic ferrules, an eccentricity of about +0.5 to about +0.1 .mu.m, usually around +0.2 .mu.m, is now secured when the outer diameter of the ferrules is usually set to be as small as about 1.5 mm.
In a construction wherein such ferrules capable of from but parallel to each other, as in the conventional optical connectors described in connection with FIGS. 1 and 2, a housing or the like for fixation of the ferrules must be provided with holes for such fixation. Such holes are poor in positioning accuracy, with the result that the accuracy of distance between the axes of adjacent ferrules arrayed is usually at least +10 .mu.m, while the best attainable accuracy is around +3 .mu.m. The accuracy is not good for optical connection.
When the accuracy of inter-axis distance is of such a degree, connection of optical fibers (represented by reference numeral 32) with an optical element-cum-photoconductive components 30 such as an optical switch, an optical coupler, an optical sensor, an optical gate or other optical integrated circuit, for example, as shown in FIG. 3, entails a poor positioning accuracy of at least +3 .mu.m for connection of the optical fibers 32 with, for example, light wave guides (represented by reference numeral 34) formed in the optical element 30, Hence the connection loss attributed to deviations of the axes of the light wave guides from the axes of the optical fibers is increased. Reference numeral 36 in FIG. 3 designates optical switch portions.
As a result of extensive investigations and experiments, the present inventors have reached a conclusion that, when use is made of a multi-ferrule structure wherein a plurality of ferrule units capable of highly accurately positioning and fixing an optical fiber are juxtaposed and fixed themselves in contact with an adjacent one(s) and in parallel with each other in the case where a plurality of optical fibers are respectively connected with a corresponding number of photoconductive components such as optical fibers for connection therewith or light wave guides formed in an optical element, the accuracy of distance between the axes of ferrule units respectively disposed on both ends of the structure can be improve, let alone the accuracy of distance between the axes of mutually adjacent ferrule units, as compared with the conventional level of such accuracy, since it depends on the eccentricity of the ferrule units themselves. The positioning accuracy for connection of the optical fibers with the photoconductive components can also be improved accordingly.
Thus, an object of the present invention is to provide a multi-ferrule structure for optical fibers which is capable of heightening the positioning accuracy for connection of a plurality of optical fibers with a plurality of photoconductive components.